Kinetics and mechanism of the reactions of quinuclidines with ethyl S- aryl thiolcarbonates

Article abstract of DOI:10.1021/jo991036g

The reactions of quinuclidines with ethyl S-(4-nitrophenyl) thiolcarbonate (NPTC), ethyl S-(2,4-dinitrophenyl) thiolcarbonate (DNPTC), and ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate (TNPTC) are subjected to a kinetic study in aqueous solution, 25.0°C, ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anion. Under quinuclidine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [N] (N is the free substituted quinuclidine) are linear and pH independent, with slope k(N). The Bronsted-type plots (log k(N) vs pK(a) of quinuclidinium ions) are linear, with slope β = 0.85 for NPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral addition intermediate (T±) is rate determining, and β = 0.54 and 0.47 for DNPTC and TNPTC, respectively, consistent with a concerted mechanism. By comparison of the reactions under investigation among them and with similar aminolyses, the following conclusions can be drawn: (i) Substitution of the 4-nitrobenzenethio group in T± by 2,4-dinitrobenzenethio or 2,4,6- trinitrobenzenethio destabilizes the tetrahedral intermediate. (ii) Quinuclidines destabilize the tetrahedral intermediate relative to secondary alicyclic amines, anilines, and pyridines. The leaving abilities of isobasic amines from T± follow the sequence pyridines < anilines < secondary alicyclic amines < quinuclidines. (iii) Quinuclidines are more reactive toward the carbonyl group of phenyl 4-nitrophenyl carbonate than that of NPTC.

Full text of DOI:10.1021/jo991036g

8298
J . Org. Chem. 1999, 64, 8298-8301
Kin etics a n d Mech a n ism of th e Rea ction s of Qu in u clid in es w ith
Eth yl S-Ar yl Th iolca r bon a tes
Enrique A. Castro,* Patricio Mun˜oz, and J ose´ G. Santos*
Facultad de Quı´mica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
Received J une 28, 1999
The reactions of quinuclidines with ethyl S-(4-nitrophenyl) thiolcarbonate (NPTC), ethyl S-(2,4-
dinitrophenyl) thiolcarbonate (DNPTC), and ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate (TNPTC)
are subjected to a kinetic study in aqueous solution, 25.0 °C, ionic strength 0.2 (KCl). The reactions
are studied by following spectrophotometrically (400 nm) the release of the corresponding substituted
benzenethiolate anion. Under quinuclidine excess, pseudo-first-order rate coefficients (kobsd) are
found. Plots of kobsd vs [N] (N is the free substituted quinuclidine) are linear and pH independent,
with slope kN. The Bro¨nsted-type plots (log kN vs pKa of quinuclidinium ions) are linear, with slope
â ) 0.85 for NPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral
addition intermediate (T() is rate determining, and â ) 0.54 and 0.47 for DNPTC and TNPTC,
respectively, consistent with a concerted mechanism. By comparison of the reactions under
investigation among them and with similar aminolyses, the following conclusions can be drawn:
(i) Substitution of the 4-nitrobenzenethio group in T( by 2,4-dinitrobenzenethio or 2,4,6-
trinitrobenzenethio destabilizes the tetrahedral intermediate. (ii) Quinuclidines destabilize the
tetrahedral intermediate relative to secondary alicyclic amines, anilines, and pyridines. The leaving
abilities of isobasic amines from T( follow the sequence pyridines < anilines < secondary alicyclic
amines < quinuclidines. (iii) Quinuclidines are more reactive toward the carbonyl group of phenyl
4-nitrophenyl carbonate than that of NPTC.
In tr od u ction
son of these reactions with those of the same substrates
with alicyclic amines and pyridines shows that the
stability of the intermediate T( formed with these amines
increases in the sequence: secondary alicyclic amines <
anilines < pyridines.6 Again, the high stability of T( for
pyridines was attributed to a lower nucleofugality from
T( than that for the other amines.6
It has also been found that pyridines are expelled more
slowly than isobasic quinuclidines (tertiary alicyclic
amines) from the zwitterionic tetrahedral intermediate
formed in the aminolysis of phenyl 4-nitrophenyl carbon-
ate.7 Nevertheless, there is no information on the relative
nucleofugalities from T( of isobasic quinuclidines, anilines,
and secondary alicyclic amines.
In the present work we undertake a mechanistic study
of the reactions of ethyl S-(4-nitrophenyl) thiolcarbonate
(NPTC), and those of DNPTC and TNPTC with quinu-
clidines with the aim to (i) shed more light on the reaction
mechanism of thiolcarbonates, (ii) assess the nucleofu-
gality of quinuclidines from T(, and the relative kinetic
stability of tetrahedral intermediates by comparison of
the title reactions with those of the same substrates with
pyridines,5 anilines,6 and secondary alicyclic amines,4,8
and (iii) examine the influence of the leaving group on
the kinetics and mechanism, by a comparison of the title
reactions among them.
It has been described that secondary alicyclic amines
are better nucleofuges from a zwitterionic tetrahedral
intermediate (T() than isobasic pyridines; this has been
deduced from the pKa values at the center of the break
(pKa°) of nonlinear Bro¨nsted plots found for the reactions
of the above two series of amines with 2,4-dinitrophenyl
acetate,1 2,4-dinitrophenyl and 2,4,6-trinitrophenyl thi-
olacetates,2 and 2,4-dinitrophenyl and 2,4,6-trinitro-
phenyl dithiocarbonates.3
On the other hand, in the reactions of ethyl S-(2,4-
dinitrophenyl) thiolcarbonate4a (DNPTC) and ethyl
S-(2,4,6-trinitrophenyl) thiolcarbonate4b (TNPTC) with
secondary alicyclic amines it has been found that the
mechanism is concerted, whereas the pyridinolysis of the
above substrates proceeds by a stepwise mechanism, i.e.,
in the latter reactions a tetrahedral T( intermediate is
formed on the reaction path.5 The concerted mechanism
with the alicyclic amines was attributed to a substantial
destabilization of T( due to the substitution of a pyridine
by a secondary alicyclic amine. This destabilization is
caused by the larger nucleofugality from T( of the latter
amines compared to isobasic pyridines.5
Recently, we have studied the reactions of anilines with
DNPTC and TNPTC, and found that the former reactions
are stepwise whereas the latter are concerted.6 Compari-
Exp er im en ta l Section
(1) Castro, E. A.; Freudenberg, M. J . Org. Chem. 1980, 45, 906.
Castro, E. A.; Ureta, C. J . Org. Chem. 1990, 55, 1676.
(2) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991, 63.
(3) (a) Castro, E. A.; Iban˜ez, F.; Salas, M.; Santos, J . G.; Sepulveda,
P. J . Org. Chem. 1993, 58, 459. (b) Castro, E. A.; Araneda, C. A.;
Santos, J . G. J . Org. Chem. 1997, 62, 126.
Ma ter ia ls. The quinuclidines or their hydrochlorides (Al-
drich) were purified by recrystallization. The substrates,
(6) Castro, E. A.; Leandro, L.; Milla´n, P.; Santos, J . G. J . Org. Chem.
(4) (a) Castro, E. A.; Iban˜ez, F.; Salas, M.; Santos, J . G. J . Org. Chem.
1991, 56, 4819. (b) Castro, E. A.; Salas, M.; Santos, J . G. J . Org. Chem.
1994, 59, 30.
(5) Castro, E. A.; Pizarro, M. I.; Santos, J . G. J . Org. Chem. 1996,
61, 5982.
1999, 64, 1953.
(7) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963,
6970.
(8) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1994,
59, 3572.
10.1021/jo991036g CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/04/1999

Reactions of Quinuclidines with Ethyl S-Aryl Thiolcarbonates
J . Org. Chem., Vol. 64, No. 22, 1999 8299
Ta ble 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Am in olysis of Eth yl S-(4-Nitr op h en yl) Th iolca r bon a te
(NP TC)a
Ta ble 3. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Am in olysis of Eth yl S-(2,4,6-Tr in itr op h en yl)
Th iolca r bon a te (TNP TC)a
b
b
102[N]tot
M
,
103kobsd
,
no. of
runs
102[N]tot
M
,
103kobsd
,
no. of
runs
amine
pH
s-1
amine
pH
s-1
quinuclidinec
8.5
8.8
9.0
9.4
9.7
10.0
8.4
8.7
9.0
7.5
7.8
8.0
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
4.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
0.101-0.458
0.206-1.07
0.364-1.96
1.57-9.47
6
6
6
6
6
6
5
6
6
6
6
6
quinuclidine
7.2c
8.0d
8.2d
9.4
9.7
10.0
8.4
8.7
9.0
7.5
7.8
2.0-15
2.0-15
2.0-15
2.0-12
2.0-12
0.5-6.0
2.0-12
2.0-12
0.2-6.0
2.0-12
2.0-12
2.0-12
0.300-1.02
0.867-3.96
1.40-7.26
63.4-347
104-470
41.5-344
29.7-203
25.2-283
7.82-240
9.34-51.4
12.9-71.2
14.3-79.8
6
6
6
6
6
6
6
6
6
6
6
6
3-quinuclinidol
3-quinuclinidol
1.74-14.3
3.67-19.3
3-chloroquinuclidine
3-quinuclidinone
0.349-1.03
0.157-2.17
0.662-3.34
0.0243-0.118
0.0311-0.171
0.0321-0.179
3-chloroquinuclidine
3-quinuclidinone
8.0
a
a
In aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
Concentration of total amine (free base plus protonated forms).
In aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
b
b
Concentration of total amine (free base plus protonated forms).
c In borate buffer (0.005 M).
c In phosphate buffer (0.005 M). In borate buffer (0.005 M).
d
Ta ble 2. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Am in olysis of Eth yl S-(2,4-Din itr op h en yl)
Th iolca r bon a te (DNP TC)a
Ta ble 4. Va lu es of p Ka for Am in es a n d kN for th e
Am in olysis of Eth yl S-Ar yl Th iolca r bon a tesa
102kN, s-1 M-1
b
102[N]tot
M
,
103kobsd
,
no. of
runs
amine
pKa
NPTC
DNPTC
TNPTC
amine
pH
s-1
quinuclidine
11.4
9.8
9.0
412
27.7
5.9
2680
287
112
8.8
6800
920
840
84
quinuclidinec
8.5
8.8
9.0
9.4
9.7
10.0
8.4
8.7
9.0
7.5
7.8
8.0
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
2.0-12
0.559-2.99
1.18-7.13
2.24-12.5
16.8-94.9
23.1-140
30.8-211
4.37-22.1
8.21-45.0
12.0-68.8
1.02-5.30
1.44-6.99
1.37-8.29
6
6
6
6
6
6
6
6
6
6
6
6
3-quinuclidinol
3-chloroquinuclidine
3-quinuclidinone
7.5
0.197
3-quinuclinidol
a
Both pKa and kN values in aqueous solution, 25.0 °C, ionic
strength 0.2 (KCl).
3-chloroquinuclidine
3-quinuclidinone
dinitrophenyl, or 2,4,6-trinitrophenyl, ko and kN are the
rate constants for hydrolysis and aminolysis of the
substrates, respectively, and N represents the substituted
quinuclidine free base. In most cases the hydrolysis term
in eq 2 was negligible compared to that for the aminoly-
sis.
a
In aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl).
Concentration of total amine (free base plus protonated forms).
b
c In borate buffer (0.005 M).
d[ArS-]
NPTC, DNPTC, and TNPTC, were synthesized as previously
reported.4,8
Deter m in a tion of p Ka . The pKa values of the different
quinuclidines were determined spectrophotometrically in the
200-220 nm range, in water, 25.0 ( 0.1 °C, ionic strength 0.2
M (maintained with KCl).
Kin etic Mea su r em en ts. These were carried out by means
of a Hewlett-Packard 8453 diode array spectrophotometer, in
water, at 25.0 ( 0.1 °C, ionic strength 0.2 M (KCl); in some
reactions borate or phosphate buffer (0.005 M) was used. The
reactions were followed at 400 nm (formation of the substituted
benzenethiolate anions).
) kobsd[EtOCOSAr]
(1)
(2)
dt
kobsd ) ko + kN[N]
The second-order rate coefficients for aminolysis (kN)
were obtained as the slopes of plots of eq 2 at constant
pH and were pH independent. These values, together
with those of the pKa of the conjugate acids of the
quinuclidines, are shown in Table 4.
With the data of Table 4 the Bro¨nsted-type plots of
Figure 1 were obtained. The plots are linear, with slopes
ânuc ) 0.85 ( 0.1, ânuc ) 0.54 ( 0.05, and ânuc ) 0.47 (
0.05 for the reactions of NPTC, DNPTC, and TNPTC,
respectively.
The magnitude of ânuc for the reaction of NPTC is in
agreement with the values of the Bro¨nsted slopes found
in the stepwise aminolysis of similar substrates when the
breakdown to products of the zwitterionic tetrahedral
intermediate (T() is the rate-determining step.1-3,5,7,8,10-15
The mechanism for this reaction is shown in Scheme 1,
where NP is 4-nitrophenyl. The step k2 should be rate
determining for these reactions, and the formation of T(
should be an equilibrium step. The aminolysis rate
constant is in this case: kN ) k1k2/k-1 ) K1k2, where K1
is the equilibrium constant for the formation of T(.
All reactions were studied under excess of the amine over
the substrate (20-fold at least). The initial substrate concen-
tration was 5 × 10-5 M.
Pseudo-first-order rate coefficients (kobsd) were found for all
reactions; for most of them kobsd was determined by means of
the “infinity” method. Only in the slowest reactions (those of
NPTC with 3-quinuclidinone) the initial rate method was
used.9 The experimental conditions of the reactions and the
kobsd values obtained are shown in Tables 1-3.
P r od u ct Stu d ies. One of the products the title reactions
was identified as the corresponding benzenethiolate; this was
achieved by comparison of the UV-vis spectra after completion
of the reactions with those of authentic samples of the above
products under the same experimental conditions.
Resu lts a n d Discu ssion
The general rate law obtained in the present reactions
is given by eqs 1 and 2, where Ar is 4-nitrophenyl, 2,4-
(9) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1987,
109, 6362.

8300 J . Org. Chem., Vol. 64, No. 22, 1999
Castro et al.
Therefore, if the k2 value is larger for 1 and the kN value
() K1k2) is larger for the reactions of PNPC, it follows
that the equilibrium constant K1 () k1/k-1) is greater for
the formation of 2 than that of 1. It is difficult to evaluate
the relative values of k-1 (rate constant for expulsion of
quinuclidines) for 1 and 2 since the larger push provided
by NPO in 2 than that by NPS in 12 should be compen-
sated by the stronger push exerted by EtO in 1 than that
by PhO in 2.14a
On the other hand, the values of ânuc ) 0.54 and 0.47
found for the reactions of DNPTC and TNPTC (Figure
1) are in accord with a concerted process where the
structure of the transition state remains constant with
the variation of the nucleophile basicity.16 These ânuc
values are in agreement with those found in the concerted
reactions of secondary alicyclic amines with DNPTC and
TNPTC, as indicated by the linear Bro¨nsted plots on-
tained, with slopes â ) 0.56 and 0.48, respectively,4
together with the fact that the predicted Bro¨nsted breaks,
had these reactions been stepwise, were not observed.4
The above ânuc values are also in accordance with the
value ânuc ) 0.54 found in the concerted reactions of
TNPTC with anilines.6
F igu r e 1. Bro¨nsted-type plots obtained in the reactions of
quinuclidines with ethyl S-(4-nitrophenyl) thiolcarbonate
(NPTC), ethyl S-(2,4-dinitrophenyl) thiolcarbonate (DNPTC),
and ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate (TNPTC), in
aqueous solution, 25.0 °C, ionic strength 0.2.
Sch em e 1
The â value alone is not sufficient to prove that a
reaction is concerted; one must be sure that the hypo-
thetical break (pKa°) of the Bro¨nsted-type plot (due to the
change in the rate determining step of a stepwise process)
is located at a pKa value within the pKa range of the
nucleophiles used in the plot.16 Unfortunately, there are
no studies on stepwise reactions of quinuclidines with
thiolcarbonates where pKa° values have been reported,
to estimate an hypothetical Bro¨nsted break for the
reactions of DNPTC and TNPTC with quinuclidines.
Gresser and J encks have found a biphasic Bronsted plot
with pKa° ) 7.5 in the reactions of quinuclidines with
phenyl 2,4-dinitrophenyl carbonate (PDNPC).7 Consider-
ing the increase of the pKa° value of 0.8 pKa unit by the
change of nucleofuge from 2,4-dinitrophenoxide to 2,4-
dinitrobenzenethiolate in the pyridinolysis of methyl 2,4-
dinitrophenyl carbonate11b and DNPTC,5 and taking into
account the fact that for the reactions of quinuclidines
with PNPC7 pKa° ) 10.7, while for those with NPTC pKa°
> 11.5 (this work), it is possible to expect a value of pKa°
> 7.5 for the reactions of DNPTC with quinuclidines. No
curvature is observed in Figure 1 for DNPTC at pKa >
7.5. This argument and the great difference found in the
Bro¨nsted slopes for the reactions of DNPTC and TNPTC
compared to that for NPTC (Figure 1), suggests that the
mechanisms of the reactions of DNPTC and TNPTC with
quinuclidines are concerted.
A similar result was found by Gresser and J encks for
the reactions of phenyl 4-nitrophenyl carbonate (PNPC)
with quinuclidines.7 A comparison between the reactions
on NPTC and PNPC with quinuclidines shows that the
kN values for PNPC are about 1 order of magnitude
greater than those for NPTC. On the other hand, a
comparison between the zwitterionic tetrahedral inter-
mediates 1 and 2 formed in the reactions of quinuclidines
with NPTC and PNPC, respectively, suggests that the
nucleofuge is expelled faster in 1 (greater k2) because
4-nitrobenzenethiolate anion is less basic than 4-nitro-
phenoxide anion (pKa ) 4.6 and 7.1, respectively),7,8 and
furthermore, the push provided by EtO in 1 to expel the
leaving group is greater than that exerted by PhO in 2.14a
(10) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
7018.
(11) (a) Bond, P. M.; Moodie, R. B. J . Chem. Soc., Perkin Trans. 2
1976, 679. (b) Castro, E. A.; Gil, F. J . J . Am. Chem. Soc. 1977, 99,
7611. (c) Castro, E. A.; Iban˜ez, F.; Lagos, S.; Schick, M.; Santos, J . G.
J . Org. Chem. 1992, 57, 2691. (d) Castro, E. A.; Valdivia, J . L. J . Org.
Chem. 1986, 51, 1668.
(12) Campbell, P.; Lapinskas, B. A. J . Am. Chem. Soc. 1977, 99,
5378. Um, I.-H.; Kwon, H.-J .; Kwon, D.-S.; Park, J .-Y. J . Chem. Res.
Synop. 1995, (S) 301, (M) 1801. Oh, H. K.; Shin, C. H.; Lee, I. J . Chem.
Soc., Perkin Trans. 2 1995, 1169.
(13) Um, I.-H.; Choi, K.-E.; Kwon, D.-S. Bull. Korean Chem. Soc.
1990, 11, 362.
It has been reported that the reactions of DNPTC and
TNPTC with pyridines5 and the former with anilines6
(14) (a) Castro, E. A.; Santos, J . G.; Tellez, J .; Uman˜a, M. I. J . Org.
Chem. 1997, 62, 6568. (b) Castro, E. A.; Cubillos, M.; Santos, J . G. J .
Org. Chem. 1996, 61, 3501. (c) Castro, E. A.; Cubillos, M.; Santos, J .
G.; Te´llez, J . J . Org. Chem. 1997, 62, 2512.
(15) Castro, E. A.; Cubillos, M.; Iban˜ez, F.; Moraga, I.; Santos, J . G.
J . Org. Chem. 1993, 58, 5400. Oh, H. K.; Woo, S. Y.; Shin, C. H.; Park,
Y. S.; Lee, I. J . Org. Chem. 1997, 62, 5780.
(16) Williams, A. Acc. Chem. Res. 1989, 22, 387.

Reactions of Quinuclidines with Ethyl S-Aryl Thiolcarbonates
J . Org. Chem., Vol. 64, No. 22, 1999 8301
k1 values for quinuclidines (which are beyond pKa 11.4)
should be greater than those for isobasic secondary
alicyclic amines. (iii) In the reactions with quinuclidines,
the center of curvature (pKa of a quinuclidine for which
k2 ) k-1) it is not observed in the pKa range studied and,
therefore, it must be at pKa > 11.4. Considering the first
and second observations and taking into account that the
value of k2 must be the same for both amine series, since
the amino moiety in T( cannot exert the “push” to expel
the leaving group,7 it is possible to conclude that quinu-
clidines are expelled faster from the T( intermediate than
isobasic secondary alicyclic amines. Therefore, quinu-
clidines destabilize kinetically the tetrahedral intermedi-
ate relative to secondary alicyclic amines. In other words,
the fact that the pKa° for the reactions of NPTC with
quinuclidines is larger than that for secondary alicyclic
amines (see Figure 2) means that the k-1/k2 ratio is
greater for quinuclidines.3b Since k2 should not change
significantly either with the basicity or the nature of the
amine,7 the larger k-1/k2 ratio for quinuclidines implies
a larger k-1 for these amines relative to isobasic second-
ary alicyclic amines.
In summary, we can conclude that quinuclidines
destabilize kinetically the tetrahedral intermediate (due
to a larger k-1) relative to the other series of amines
studied. Therefore, the sequence: quinuclidines > sec-
ondary alicyclic amines > anilines > pyridines seems to
hold for the rate constants for amine expulsion (k-1) from
the zwitterionic tetrahedral intermediate.
By comparison of the concerted reactions of the series
of quinuclidines with DNPTC and TNPTC with the
stepwise mechanism for the reactions of the same amines
with NPTC, it can be concluded that there is a remark-
able destabilization of the tetrahedral intermediate by
introducing a second and a third nitro group to the
nucleofuge. This destabilization must arise from the
lower basicity of TNPS and DNPS, relative to NPS (pKa
1.4, 3.4, and 4.6, respectively)2 which results in a larger
nucleofugality of the former two groups from the inter-
mediate T( and renders these species highly unstable
kinetically. The intermediate either no longer exists and
the concerted mechanism is enforced, or it is so unstable
that the reaction bypasses it and proceeds through a
lower energy path (the concerted route).17
F igu r e 2. Bro¨nsted-type plots (statistically corrected)2 ob-
tained in the reactions of ethyl S-(4-nitrophenyl) thiolcarbonate
(NPTC) with quinuclidines (b, this work) and secondary
alicyclic amines (2, ref 8), in aqueous solution, 25.0 °C, ionic
strength 0.2.
proceed through the formation of a tetrahedral interme-
diate (T(). The fact that the reactions of the same
substrates with quinuclidines are concerted (this work)
can be explained by a faster expulsion rate from T( of
the latter amines than isobasic pyridines and anilines.
Namely, quinuclidines destabilize kinetically the tetra-
hedral intermediate relative to pyridines and anilines.
Figure 2 shows the Bronsted-type plots for the reac-
tions of NPTC with quinuclidines (this work) and second-
ary alicyclic amines.8 Three observations can be made:
(i) In the pKa range where the second step is rate
determining the kN () k1k2/k-1) values are greater for
secondary alicyclic amines than for isobasic quinuclid-
ines. (ii) Comparison of the kN values in the high pKa
region (where the k1 step is rate limiting for the reactions
of the secondary amines) allow us to conclude that the
Ack n ow led gm en t. We thank FONDECYT of Chile
for financial assistance to this work.
J O991036G
(17) J encks, W. P. Acc. Chem. Res. 1980, 13, 161. J encks, W. P.
Chem. Soc. Rev. 1981, 10, 345. Williams, A. Chem. Soc. Rev. 1994, 23,
93.